1. Field of the Invention
The present invention relates to a photothermographic imaging material.
2. Description of the Related Art
In Recent years, in the field of medical services, reduction of processed waste fluids has been intensively desired in view of environmental protection and space saving. In this concern, a technology in respect of photosensitive thermal development photograph materials used for medical diagnoses and photographic techniques, which can be exposed efficiently by a laser image setter or a laser imager and can form black images provided with high resolutions and definition, has been required. With such photosensitive thermal development photograph materials, it becomes needless to use solution-type processing chemicals, and it enables the users to be implemented with a thermal development process system that is simpler and does not destroy the natural environment.
On the other hand, the semiconductor laser technology having been progressing in recent years has allowed image output apparatuses for medical use to be compact in their sizes. Naturally, a technology in respect of infrared-sensitive thermal development photograph materials capable of utilizing semiconductor laser as a light source has been developed also, and various spectro-sensitization technologies have been disclosed (see JP Hei-3-10391B, JP Hei-6-52387B, JP Tokukaihei-5-341432A, JP Tokukaihei-6-194781A, JP Tokukaihei-6-301141A).
However, in the photothermographic imaging materials with which recordings are achieved by using a laser light source, it is desired that the material can greatly absorb light in accordance with a bright-line wave length. However, colorants each having its maximum absorption band in a region of red to near infrared have long conjugate chains and are inclined to receive influences by the environment. As a result, though the colorant has a high molar absorption coefficient in a solution, there happens easily such a phenomenon that the colorant disappears during the preparation, or that the colorant takes a number of conformations and forms a wide absorption spectrum with small maximum absorption intensity in a photosensitive material. Further, the interval between the lowest vacant level and the highest occupied level is narrow, and the lowest vacant level and the highest occupied level are approximate to the conduction band of silver halide particles. Therefore, there happens such a problem that fogging can be caused easily, or that the sensitivity can be lowered.
Hence, it has been desired to provide an infrared-sensitive photothermographic imaging material having a photosensitive spectrum that intensively absorbs light in a region of red to near infrared and an infrared-sensitive photothermographic imaging material that is highly photosensitive, causing less fogging and less deterioration in the performance even after a substantial time elapse.
In order to resolve the aforementioned problems, various techniques in connection with infrared sensitizing colorants and those for combining infrared sensitizing colorants have been disclosed (see U.S. Pat. No. 3,582,344, U.S. Pat. No. 4,975,362, U.S. Pat. No. 5,013,642, EP 821811, JP Tokukaihei-9-510022A, JP Tokukaihei-5-72659A, JP Tokukaihei-9-292673A, JP Tokukaihei-9-166844A, JP Tokukaihei-9-281631A, JP Tokukaihei-9-281639A, JP Tokukaihei-9-288327A, JP Tokukaihei-10-73900A, JP Tokukaihei-10-123663A, JP Tokukaihei-10-123665A).
However, it has been noted that the photosensitivity is not sufficient with such infrared sensitizing colorants as disclosed in those references, and that there is a problem of less photosensitivity and increasing fogging at the time of development and during the preservation lasting for a substantial time elapse. Hence, it has been desired to be improved.